Flat panel displays have become popular for use in outdoors due in large part to their portability. However, bright lights, and sunlight in particular, have been a persistent source of problem for the flat panel color display screens. Even a modest sunlight may "wash out" a display so as to make it practically unreadable.
Bright lights wash out displays by reflecting off of a display screen. These reflections occur at all wavelengths of the visible light hitting the screen. At the same time, light is being emitted from the screen to the viewer. Light is also being reflected off the screen. This light also has certain wavelengths corresponding to the color being emitted from the sub pixel. The bright sunlight washes out the displays by obscuring the desired color of light emitted from the display.
Conventional display screens have attempted to address the washing out problem by applying color pigment crystals into a pixel structure on the display screens. These color pigment crystals form a color filter to absorb unwanted light and transmit light of a desired color (i.e. wavelength). Specifically, color pigment crystals have been deposited into the sub-pixels of display screens in various arrangements. The color of the pigment crystals so deposited corresponds to the color of the light assigned to a sub-pixel. The conventional methods then used photolithographic process to cure the color pigment crystals into the pixels on the display. Schematic side sectional views depicting conventional steps used in depositing color pigment crystal on the pixels are shown in Prior Art FIGS. 1A through 1G. In the prior art Figures, two prior art processes are shown. In one prior art method, color pigments are used to coat the phosphors. In the other prior art method, color pigments are placed on the display screen and exposed from the interior side of the display screen.
Prior Art FIG. 1A illustrates a top view of a black matrix 102 defining a pixel structure. Black matrix 102 is disposed on the interior surface of a display screen 100, typically comprised of glass. The pixel structure is further divided into sub-pixels, typically shown as 104, of three primary light colors: red (R), green (G), and blue (B).
A side sectional view of display screen 100 with black matrix 102 disposed on it interior surface is depicted in Prior Art FIG. 1B.
Prior Art FIG. 1C illustrates a side sectional view of black matrix 102 disposed on top of display screen 100 with colored phosphors 106 disposed thereon. On the interior surface of display screen 100, plurality of sub-pixels are disposed. Phosphors 106 are first coated with color pigment crystals 104 of red, green, or blue color. Subsequently, phosphors are put into a slurry consisting of poly vinyl alcohol, a sensitizer (metal dichromate), a dispersant, a surfactant, an emulsifier, a short chain cross linker (TEG), and water. The slurry is spread across the screen. The screen is either spun (curved or flat screen) or squeegeed to remove excess slurry after setting for several minutes. It is then dried. The specific color pixel area is then exposed through a photomask using from side exposure. The excess is then removed using a water solution.
With reference to Prior Art FIG. 1D, phosphors 106 coated with color pigment crystals are then cured into corresponding pixels on the interior surface by an ultra-violet (UV) light 108 illuminated from the interior side of the display screen 100. During the UV exposure process, a photomask is used to ensure curing of color pigment crystals in desired sub-pixels only. That is, the material must not be cured along the top or the edge of black matrix 102.
However, coating individual phosphor particles with color pigment crystals produces unwanted side effects. The main function of the phosphors is to generate light when a beam of electrons bombard them. With an added coat of color pigment crystals, some electrons of lower kinetic energy may no longer be able to penetrate the added color coating. This is due to the nature of FEDs field emission display having low operating voltages and, hence, sub-micron penetration depth of electrons into the solids. Moreover, even if the electrons make it through the coated crystals, they may not have sufficient kinetic energy left to enable phosphor particles to generate light. Hence, phosphors do not generate as much light as they would without the coating. Such electron impedance translates directly into reduced brightness of the display screen images. As a result, coated phosphors require higher kinetic energy electron requiring higher voltage emitters. Consequently, coated phosphor methods have not been suitable for a low voltage class of display structures.
With reference to Prior Art FIG. 1E, a side sectional view of another display screen 100 is illustrated. On its interior surface, a black matrix 102, defining a plurality of sub-pixels corresponding to red, green, and blue colors is disposed. Subsequently, phosphors are put into a slurry consisting of poly-vinyl alcohol, a sensitizer (metal dichromate), a dispersant, a surfactant, an emulsifier, a short chain cross linker (TEG), and water. The slurry is spread across the screen. The screen is either spun (curved or flat screen) or squeegeed to remove excess slurry after setting for several minutes. It is then dried. The specific color pixel area is then exposed through a photomask using from side exposure. The excess is then removed using a water solution.
As illustrated in Prior Art FIG. 1F, the color crystal pigments 104 are then cured into the pixels on the interior surface by UV light 108 illuminated from the interior side of the display screen 100. Phosphors are then deposited into the sub-pixels on top of the color crystal pigments by a well known photolithographic process. During the UV exposure process, a photomask is used to ensure curing of color pigment crystals in desired sub-pixels only. That is, the material must not be cured along the top or the edge of black matrix 102.
Consequently, conventional methods of curing color pigment crystals require precision in mask alignment using pixelated photomasks and thus are error prone. For example, a slight misalignment of a photomask leads to curing of color pigment crystals in unwanted areas such as along the top or the edge black matrix 102. Prior Art FIG. 1F illustrates portions 109 of color pigment crystals 104 on top of black matrix 102, which should not be cured.
Those skilled in the art will no doubt appreciate that a good adhesion of the color pigment crystals to the surface of a display screen is important for reducing reflection of unwanted lights and increasing transmission of phosphor lights. Without a satisfactory adhesion of the color pigment crystals to display screens, reflectivity and transmission properties of a display screen may be adversely affected.
Unfortunately, under the conventional method shown in FIG. 1F, the color pigment crystals are often not adequately cured onto display screens due to their intrinsic nanocrystalline properties. For instance, as illustrated in Prior Art FIG. 1G, when color pigment crystals 104 are subjected to a UV light 108, a cross-linking depicted by structure 110 may occur in the crystals at the top of the layer. The color pigment crystals 104 at the top of the color crystal layer bond to each other before the color pigment crystals 104 below can satisfactorily adhere to display screen 100. Hence under conventional methods, a strong adhesion of color pigment crystals 104 to display screen 100 is not easily obtainable.
Furthermore, the cross-linking of color pigment crystals means that the exposure to an UV light must proceed at a relatively slow pace. That is, rapid exposure to UV light enhances the detrimental cross-linking effect at the top of the layer of color pigment crystals.
Thus, a need exists for a method to create a color filter on a display screen which improves readability of display screens by reducing reflection of ambient light and by increasing transmission of phosphor light of desired wavelength without reducing generation of light by phosphors. A further need exists to achieve the above-mentioned color filter in a way that provides a more efficient adhesion of color pigment crystals to the screen display structure while decreasing the need for high precision alignment thereby decreasing equipment cost.